Essentially what you are arguing is that we can't dare make parry useful or else tanks might have to sacrifice damage for mitigation? It may blow your mind but a ton of people are perfectly ok with that. This isn't something that hasn't occurred to people proposing parry changes, for many it's an inherit benefit as a large number people believe tanks should be about mitigation first and foremost and the obsession with dps many tanks have is contrary to their role.

It wouldn't eliminate strength builds either, it'd simply make non-dps focused set ups more relevant, and actually increase potential gear choice (parry is on more than just right side gear). Even in your 10% over the course of a fight example, there are plenty of people who would still choose the increased damage. Choosing strength over vitality already introduces extra stress on your healers, as do things such as stance dancing while tanking, but these are acceptable sacrifices some people make.